It is well known in the electronic arts to manufacture pressure sensors using semiconductor substrates. Silicon is the most commonly used semiconductor material although other piezo-resistive and piezo-electric materials can also be used.
In a typical semiconductor pressure sensor, a piezo-resistive region is formed in or on the semiconductor surface. A current is passed through the resistor and the voltage developed across the resistor is measured. The resistor is generally oriented so that the largest piezo-resistive effect is obtained when the silicon substrate is bent in response to the applied pressure or force. In order to increase the sensitivity of such devices, the piezo-resistive region is typically formed in or on a portion of the silicon substrate which has been made much thinner, e.g., an etched diaphragm or beam. The diaphragm or beam is usually some regular geometric shape, e.g., a circle, square, ellipsoid, rectangle, etc. Diaphragm type pressure sensors and methods for producing them are described in U.S. Pat. Nos. 4,783,237, 4,672,354, 4,670,969, 4,317,126, 4,275,406, 4,204,185, 3,994,009, 3,968,466, 3,767,494, 3,758,830, and 3,230,763 which are incorporated herein by reference.
The sensitivity of such semiconductor devices depends, among other things, on the crystallographic orientation of the substrate, the direction of current flow, and the thickness and lateral size of the diaphragm. Generally, the design of such pressure sensors requires a balancing of conflicting or competing requirements, including those associated with manufacturing, device size, cost and the like, in order to obtain the most useful result.
Among the most difficult operations required during manufacture of such sensors is the etching of the diaphragm since the diaphragm thickness and size crucially affect device performance and manufacturing yield. In a typical device, a doped resistor region is formed on the substantially planar front face of a semiconductor wafer. Then, approximately ninety percent or more of the thickness of the wafer immediately behind the resistor region is removed by etching a cavity inward from the opposed rear face of the wafer to form the pressure sensing diaphragm. Silicon having {100} orientation is much used for manufacturing pressure sensors because of its known anisotropic etching characteristics that assist in controlling the thickness and shape of the silicon diaphragm.
Silicon pressure sensors formed in this fashion are comparatively easy to make, have generally good sensitivity and repeatability, and are sufficiently rugged to be useful in a variety of applications. Means and method for their manufacture are well known in the art.
In many applications, the pressure to be measured is applied to the front (planar) face of the pressure sensor while the rear (cavity) face is the reference side. This arrangement is particularly common where pressure differentials above atmospheric pressure are to be measured. However, there are an increasing number of applications where it is desirable to be able to pressurize the cavity side of the diaphragm, and with pressures approaching ten atmospheres or more. The higher the applied pressure, the greater the stress on the diaphragm and the greater the incidence of diaphragm rupture.
It has been found that anisotropically etched pressure sensors exhibit asymmetrical over-pressure failure, that is, the diaphragm rupture pressure is much smaller when the pressure is applied to the rear (cavity-side) face of the diaphragm than when applied to the front (planar-side) face of the diaphragm. This is highly undesirable, since it makes it much more difficult to build anisotropically etched pressure sensors that combine good sensitivity with adequate rear-face diaphragm rupture capability.
Accordingly, it is an object of the present invention to provide an improved means and method for forming etched cavity type devices. It is a further object of the present invention to provide an improved means and method for diaphragm type pressure sensors which increases the rearface (cavity-side) diaphragm failure pressure.